Introduction
With the ever increasing population and rapid economic
growth, the need for land for development as well as the need for
transportation development as a means of connecting between cities will also
increase rapidly. This results in the unavoidable use of land that is
structurally unfavorable condition. The limited land available for the location
of the construction of transportation facilities has become a problem in
itself. Therefore, the unfavorable structural condition of the land should not
be a problem that can hinder the development of transportation facilities.
The construction of transportation facilities on soft soil
will cause problems where the bearing capacity of soft soil is low and the
process of land subsidence will require a long period of time. As is known, the
land subsidence factor causes many problems in the process of building
transportation facilities, besides that, there will also be problems at the
level of service (serviceability). Therefore, this problem must be anticipated
before the transportation facility begins to function.
There are many methods that can be done as shown in Figure
1. Where in the selection of methods, the efficiency and effectiveness factors
are very dominant. One method that can be done is to fill the soil combined
with vertical drain and horizontal drain. This method was chosen to be one of
the most widely used methods because it is easy to do and quite efficient. The
following is a list of soil improvement methods that can be performed on soft
soil.
Figure 1
Soft soil improvement
method (Kuswanda,
2016)
Soil improvement using embankment combined with vertical
drain and horizontal drain is a relatively practical method compared to other
methods. This is due to the ease of implementation and relatively cheap in
terms of cost. In addition, the location conditions which are difficult to
obtain sand (materialsand blanket) cause the use of horizontal drain to be an
alternative to sand blanket. The purpose of stockpiling the soil is to
consolidate the soil as a bearing stratum so that it consolidates first and
increases the shear strength (gain of strength) of the soil before the soil is
given the actual final load. The process of loading for landfilling can use
various methods of loading, one of which is the method of loading in the form
of an embankment.
In
the combined landfill method With PVD (prefabricated vertical drain) and PHD
(prefabricated horizontal drain), soil improvement is carried out by placing
embankment loads on the subgrade in accordance with thework loadandconstruction
loadplanned. The duration of loading is carried out until the planned
consolidation. If the degree of consolidation of the subgrade has reached the
planned value, road construction work can begin.
Figure 2
Soil filling system
with PVD and PHD���� (Kuswanda,
2016)
From Figure 2 it can be understood that soil filling is
carried out to compress the soil so that it undergoes consolidation first and
increases the shear strength of the soil before the soil is given the actual
final load. To avoid failure during soil filling, the embankment load can be
planned so that in the end the original soil will be able to carry the service
load and avoid excessive settlement.
Soil improvement by means of stockpiling soil and vertical
drain can be combined, where landfill will function as a medium for
compression, while vertical drain will function as a medium for eliminating
excess pore water stress. Furthermore, the combination of soil stockpiling and
PVD results in the consolidation process going fast, so that the planned land
subsidence will be in accordance with the soil compaction plan according to the
age of the project. This can be seen in Figure 1.3.
Figure 3
Overview of backfill
combined with vertical drain (Maryono,
2011)
Repair of soft soil with soil filling combined with PVD is
a soil improvement system consisting of load-bearing work, PVD, PHD and
geotechnical instruments. Backfill serves to apply pressure to the subgrade.
PVD serves to accelerate the process of soil compaction. PHD functions to drain
water from the PVD in a horizontal direction to the outside of the site where
the soil is undergoing improvement. Meanwhile, geotechnical instruments
function to monitor the process and determine the performance of the results of
soil improvements carried out.
The process of loading with soil embankment will serve to
shorten the length of drainage so that it will reduce the consolidation time as
shown in Figure 4. In its application there are various methods of soil
improvement that can be done. Of the many methods, themethod Prefabricated
vertical drain (PVD) is currently very commonly used in various engineering
work. The PVD is plugged into the soil to be repaired, in this case usuallysoil
undrained or soil with very low permeability so that excess pore water due to
loading with soil piles can flow into the PVD. As it is known that the function
of PVD is to shorten the length of drainage, a number of PVDs are needed in the
area to be repaired (Holtz,
1987).
Figure 4
Cross section of soil
loading (Holtz,
1987)
Prefabricated Horizontal Drain (PHD) is starting to be
commonly used as an instrument to accelerate the soil compaction process which
is faster than the use of Preloading and Vertical Drain (Feng,
J., Ni P., Chen Z., Mei G., 2020). This horizontal drain
actually replaces the function of sand (sand blanket) with an installation
pattern of 1 PVD: 1 PHD so as to avoid bending the PVD tape. So that the water
from the PVD filter can enter almost completely into the PHD filter.
The success of the horizontal drain is only if the water
that comes out is equal to the total water carried by the PVD minus the volume
of the cavity contained in the PHD. Because the principle of PHD releases
water, namely by filling the PHD cavity with water from the PVD so that the
water will flow out through the edges by itself. This means that there must be
a difference in total head so that water can flow. So when the volume of water
is equal to the volume of the PHD stretch, then at that level the total head
does not have a difference. This means that water will still settle in the PHD
as large as the PHD cavity in conditions after soil subsidence due to loading
with soil piles (Anjana et al. 2020).
In this study, an analysis will be carried out to
determine the level of subsidence of the soft soil layer at STA 103 + 550 on
the Tebing Tinggi-Kisaran Toll Road Development Project which has been improved
by applying a load technique using soil stockpiling combined with PVD and PHD
using analytical methods and numerical methods using PLAXIS 3D. Furthermore,
the results obtained from the subsidence of soft soil will be compared with
field monitoring using theinstrument settlement plate as a control for the
modeling carried out.
Method
A. General
Project Data The project
Location
in this research is the Tebing Tinggi � Kisaran Toll Road Development Project
Phase 1 carried out by PT. Hutama Karya at STA 103 + 550. The review carried
out will refer to the field monitoring process using theinstrument settlement
plate 139which is installed in the middle of the cross-section of the road
where the soft soil will be repaired using the method of giving the soil
embankment load combined with prefabricated vertical drain and prefabricated
horizontal drain instead of sand blanket. The description of the location used
as research is shown in Figure 5 below.
Figure 5
Map
of the research location
To find out the repair process that will
be carried out at STA 103 + 550, it is necessary to have a soft drawing of the
improvement plan that will be carried out at the research location. So that the
right modeling process can be chosen to get a modeling process that is close to
the conditions in the field. The soft drawing of the management plan for the
improvement of soft soil at STA 103 + 550 is shown in Figure 6.
Figure
6
Cross
section STA 103 + 550
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B. Overview
of Soil Conditions
In writing this thesis, the author
analyzes soil conditions based onsoil data Bore Hole-40which represents the
surrounding soil conditions. This bore hole was chosen because it is the
closest test point to the research location, namely STA 103 + 550. The data
obtained from this bore hole will later be used as the basis for determining
the soil parameters to be used in 3D PLAXIS modeling.
Determination of soil conditions is based
on the results of f ield soil test data and then a line is drawn on the type of
soil and soil characteristics that are considered the same or have
similarities. The selected test point conditions are considered capable of
representing the soil parameter values for each layer in this STA
103 + 550. Determination of soil conditions is supported by 2 laboratory
testing processes carried out on 2 different types of soil layers, namely at a
depth of 3.5 � 4 meters and a depth of 11.5 � 12 meters. And for the
determination of soil parameters in the soil layer that was not carried out
laboratory testing was carried out with a correlation approach based on the
N-SPT value data obtained from the Table Bore Log as shown in Figure 7.
Figure 7
Boring Log
From Figure 7, several soil parameters
will be determined that will be used as the basis for modeling in PLAXIS 3D.
Determination of these soil parameters will also refer to the results of
laboratory test research conducted on several layers of soil contained in bore
hole 40. And for several other soil layers that are not subjected to laboratory
testing, correlations are carried out using the references described in Chapter
II.
The following are the results of
laboratory tests carried out for soil layers at a depth of 3.5 � 4 meters and a
depth of 11.5 � 12 meters are shown in Table 3.1. And due to the limited data
available from laboratory test results, the selection of the soil model that
will be used in the PLAXIS 3D modeling is to use thesoil model Mohr-Coloumb. It
is expected that the modeling value with thesoil model Mohr-Coloumb carried out
in this study is close to the data value of the actual conditions that occur in
the field which are monitored by monitoring using a settlement plate. To
support the writing of this thesis, the authors obtained data from the
contractor, namely PT. Waskita Karya. The data obtained are:
a)
Job layout,
b)
Boring Log
c)
Soil parameter data
d)
PVD and PHD specifications
e)
Geotechnical instruments (settlement plate)
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Table 1
Laboratory
test results
C. Overview
of Handling in the Field
In the soil improvement process using the
soil filling method used at this location, it is necessary to use several
approaches in the modeling process which will later be used in the Plaxis 3D
program. This approach needs to be done to be able to model the value of the
decline that occurs in the field. Considering the conditions in the field, of
course, some adjustments will be made that are deemed necessary to save costs.
For example, the use of 1 PHD to connect 2 PVDs can be done because at the time
of use in the field the horizontal flow value is not disturbed by the use of
this method. Of course, during modeling this will be quite difficult because in
PLAXIS 3D modeling line drains can only be used to connect two points.
Figure
8
Installation
of PVD and PHD in the field
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Figure 8 shows the work process carried
out in the field. Where in the field work the use of PHD is more optimized by
using 1 line of PHD to flow 2 lines of PVD. This can be done because in the
previous soil improvement process, when trying to optimize the use of PHD,
there was no blockage in the horizontal flow.
D. Stages
of Research
In writing this thesis, the authors carry
out several stages of implementation in order to achieve the aims and
objectives of the research, as summarized in Chapter I. To facilitate the
achievement of these goals, the authors perform the following stages:
1.
The first; stage is to collect various types
of literature in the form of books and scientific writings related to this
thesis.
2.
The second; stage is to collect data that will
be used in analyzing. The subject of this thesis is the construction of the
Tebing Tinggi - Kisaran Toll Road. The data needed for writing this thesis was
obtained from PT. Hutama Karya as contractor. The data obtained are the results
of the SPT (Standard Penetration Test), soil parameters, PVD and PHD
specifications and geotechnical instruments.
3.
The third; stage At this stage, activities
will be carried out to analyze the settlementsettlement ofof the soft soil
layer which is improved by the method of applying the load using soil piles
combined with PVD and PHD using PLAXIS 3D. This analysis process will use the
basic modeling method Hird (1995) which performs analysis using the
Mohr-Coloumb soil model to model the soil in the PLAXIS 3D program so that it
is expected to be close to soil conditions that have improved in the field.
4.
Fourth; stage At this stage, activities are
carried out to analyze the decrease in total settlement of soft soil layers repaired
with PVD and PHD using the PLAXIS 3D program to then be compared with field
monitoring data obtained from theinstrument settlement plate.
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Furthermore, the research flow chart is
attached in Figure 9 in order to make it easier to understand the steps taken
by the author in this study.
Figure 9
Research Flowchart
General
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Result
and Discussion
A. The
results of the analytical calculation obtained the amount of land subsidence of
16.09 cm. This result is relatively close to the actual decrease observed in
the field, where the decrease is 16.3 cm. There is a difference of 0.21 cm
because the laboratory data do not represent the entire soil layer in the
field, where from the boring data only 2 samples of laboratory testing were
carried out, namely at a depth of 3.5 - 4 meters and a depth of 11.5 - 12
meters.
B. After
analyzing using plaxis 3d modeling with several approaches, the results are
obtained as shown in table 4.3. From the results of this approach, it can be
concluded that the modeling value with 2 pvd lines and 2 phd lines taking into
account the smearzone effect is closer to the decrease in results from field
monitoring.
C. The
modeling approach used to determine the soil parameters that were not carried
out by laboratory testing obtained results close to the results of observations
in the field, with a percentage value of < 1.5%.
D. The
modeling approach, although different from the work in the field, obtained
results that were quite close to the data from the monitoring carried out in
the field. This shows that phd as horizontal drains can drain 1 line of pvd or
2 lines of pvd.
E. The
pattern of decline shown in figure 4.9 illustrates that the predicted decline
obtained from plaxis 3d is larger and faster than the decline observed in the
field. This is because the mohr-coloumb soil model only uses one value of the
soil stiffness modulus (e). From the graph it can be concluded that the e value
that occurs in the field is smaller than the modeled e value. However, if in
plaxis 3d the value of e is reduced, then the results obtained will be greater
than the results obtained in the field. This can be anticipated by using soil
modeling with the hardening soil model which can model the secant stiffness and
tangential stiffness of the soil.
F. The
depiction of the smear zone effect with the disturbed zone value described as 2
x the mandrel dimension and the k value of disturbed soil is 2 times smaller
than the initial k of the soil, obtaining results that are quite close to the
results of monitoring in the field. This shows that the modeling that has been
done is appropriate. Due to the reality on the ground, the soil will experience
a smearzone effect as a result of the pvd erection process on a 17.4 meter long
soil layer.
G. Phd
is proven to function as horizontal drains to replace the sand blanket
function. This can be used as an alternative in the future for the improvement
of soft soil with the method of loading with soil piles combined with pvd,
which is difficult to access sand as a sand blanket material can use phd.
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Conclusion
Based on the results of the analysis and discussion, it
can be concluded that: 1). Calculation
of consolidation settlement using analytical theory is close to the actual
settlement results that occur in the field with a value of 16.09 cm compared to
field monitoring of 16.3 cm with a percentage difference of 1.31% . This difference
in settlement occurs because the laboratory data values cannot
represent the entire soil layer under review.
2). The decrease in consolidation using plaxis 3d modeling with 2 lines
of pvd and 2 lines of phd and taking into account the effect of the smear zone
resulted in the most accurate calculation model with a value of 16.28 cm
compared to field monitoring of 16.3 cm with a percentage difference of 0, 12%. 3). Phd works well as a substitute for
sand blanket material to drain water horizontally from the end of the pvd to
leave the repaired soil location. With this, phd can be used as an alternative
to horizontal drains in soil improvement with the loading method using soil
heap combined with pvd, if the location is difficult to find sand material as a
sand blanket. 4). The pattern of
decline that occurs in the 3d plaxis modeling is greater in a shorter time than
the decline that occurs in the field. This is because in this study modeling
was carried out using the mohr-coloumb soil model. This can be corrected by
soil modeling using a hardening soil model, with a more complete laboratory
test value.
References
Feng, J.,
Ni P., Chen Z., Mei G., Xu M. (2020). Positioning Design of Horizontal Drain
in Sandwiched Clay-Drain Systems for Land Reclamation.
Holtz, R. D. (1987). Preloading with
Prefabricated Vertical Strip Drains, School of Civil Engineering, Purdue University
Indiana, USA.
Kuswanda, W. P. (2016). Perbaikan Tanah
Lempung Lunak Metoda Preloading pada Pembangunan Infrastruktur Transportasi di
Pulau Kalimantan, Prosiding Seminar Nasional Geoteknik.
Maryono, O. (2011). Analisis Perbandingan
Karakteristik Penurunan dengan Metode Analitik, Numerik dan Monitoring Lapangan
Pada Perbaikan Tanah dengan Sistem Preloading Dikombinasikan PVD pada Area Bore
Hole II Apron Bandara Kuala Namu.
